Embodiments of the present invention relate generally to power conversion systems, and in particular, to power conversion systems applied in electric vehicles or the like.
In recent years, there has been much research focused on countermeasures for environmental pollution and fossil fuels exhaustion. In particular, the automotive industry has concentrated efforts in the research of electric vehicles (EV) using batteries as the main energy source. In general, EV systems need a power conversion system which is used to transfer energy from the battery to a traction motor for driving the electric vehicle.
For example, FIG. 1 shows a conventional power conversion system 10 of an electric vehicle. The power conversion system 10 may include a battery 11, a traction motor 12, a DC/DC converter 13, a three-phase inverter 14, a DC link 15 including a capacitor C1, a filter unit 16, and a control unit 17. The DC/DC converter 13 is used to boost DC voltage of the battery 11 and is electrically coupled to the battery 11 through the filter unit 16. The inverter 14 is used to convert the boosted DC voltage from the converter 13 to three-phase AC voltage through the DC link 15. The traction motor 12 receives the converted three-phase AC voltage from the inverter 14 to drive the electric vehicle. The control unit 17 is used to provide pulse-width modulation (PWM) commands to the converter 13 and the inverter 14, to convert the DC voltage to three-phase AC voltage accordingly.
For implementing voltage conversion, the converter 13 and the inverter 14 each may include multiple switch elements such as twelve switch elements S1-S12 controlled by the PWM commands of the control unit 17. In the conventional power conversion system 10, these switch elements S1-S12 may include transistors such as silicon insulated gate bipolar transistors (IGBTs). However, the traditional silicon IGBTs S1-S12 may consume lots of energy during the energy transfer process, which decreases the efficiency. On the other hand, due to the silicon IGBTs S1-S12 generating a lot of heat, heat sinks (not shown) need to be arranged on the silicon IGBTs Sl-S12. This requires additional space and increases the weight of the electric vehicle, which in turn reduces its power density and performance.
Furthermore, for achieving multiple phases of voltage, the filter unit 16 may include multiple inductors, such as three interleaved inductors L1, L2, and L3. However, these individual inductors L1, L2, and L3 may require additional space of the electric vehicle as well.
In addition, for controlling these switch elements S1-S12, the control unit 17 should provide proper PWM commands. For example, FIG. 2 shows a control schematic diagram of the control unit 17 for controlling the DC/DC converter 13. The control unit 17 may include a difference element 171 and a PWM command generation block 172. The difference element 171 is used to obtain a DC voltage error signal VDC_Err calculated from a difference between a feedback DC voltage signal VDC_Fbk on the capacitor C1 and a predetermined DC voltage command signal VDC_cmd. The PWM command generation block 172 is used to generate proper PWM commands (PWM_cmd) according to the DC voltage error signal VDC_Err. The feedback DC voltage signal VDC_Fbk stands for the real measured voltage on the capacitor C1 and the predetermined DC voltage command signal VDC_cmd stands for a predetermined DC voltage.
It is understood that the PWM command generation block 172 may include some calculating elements, such as proportional integral regulators, limiters, difference elements, comparators, etc., to calculate the PWM commands (PWM_cmd). In the conventional control unit 17, the predetermined DC voltage command signal VDC_cmd is a constant voltage value (see FIG. 2). However, because the predetermined DC voltage command signal VDC_cmd is a constant voltage value, and the DC/DC converter 13 outputs DC power while the inverter 14 outputs AC power, the capacitor C1 should have sufficient capacitance to handle the pulsation power.
For these and other reasons, there is a need for embodiments of the invention.